Heart rate variability (HRV) refers to the changes in the length of time between consecutive heart beats during sinus rhythm and is primarily due to the interaction between the sympathetic and parasympathetic arms of the autonomic nervous system. Measurement and analysis of heart rate variability is thus a useful and non-invasive tool for assessing the status of the autonomic nervous system.
The interval between successive heart beats, referred to herein as a BB interval, may be measured from a surface ECG or intracardiac electrogram as the time from the peak of one R wave representing ventricular depolarization to the peak of the next, referred to as an RR interval, or as the time from the peak of one P wave representing atrial depolarization to the peak of the next, referred to as an PP interval. Other measures of BB interval are possible and the present subject matter is not limited to the measures of BB interval based on morphological peaks described above. The variability of normal BB intervals (i.e., during sinus rhythm) can be determined and analyzed in several different ways in either the time domain or the frequency domain. Time domain measurements involve the computation of statistics based upon the individual BB intervals making up a BB time series such as the standard deviation of the BB intervals in the series. Frequency domain analysis employs methods such as the Fast Fourier Transform (FFT) or autoregressive analysis to analyze the frequency spectrum of the variability in the BB intervals. Analysis of the frequency spectrum of heart rate variability has proven to be particularly valuable in assessing the relative activities of the sympathetic and parasympathetic nervous systems in a subject. Such assessment of the state of autonomic balance is a useful function for implantable cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators to perform as it can be used to modify the manner in which therapy is delivered by the device or to predict the occurrence of arrhythmias.
Heart rate variability is a useful indicator of a subject's physiological state because it reflects the influence of internal feedback mechanisms within the body on the intrinsic heart rate produced at the sino-atrial node of the heart. Such feedback mechanisms are based on the interplay of the sympathetic and parasympathetic branches of the autonomic nervous system as well as various hormonal responses. Ectopic cardiac activity, such as premature atrial contractions (PAC's) and premature ventricular contractions (PVC's), are believed to be independent of the aforementioned internal feedback mechanisms. A heart rate variability metric calculated from a BB time series in which ectopic beats are present will be corrupted and may not accurately reflect the subject's true physiological state. For purposes of heart rate variability analysis, it is therefore desirable to exclude those BB intervals from a BB time series which are due to ectopic cardiac activity and are not the result of a normally conducted heart beat. It is with this problem that the present invention is primarily concerned.